1. Field of the Invention
The present invention relates to an image processing apparatus and, more particularly, to an image processing apparatus which processes coded data.
2. Related Background Art
In a system for synchronously reproducing a video image and an audio signal as an example of such an image processing apparatus, there are systems such as a TV conference system, a television telephone system, or the like which require a real-time performance and a system in which a real-time performance is relatively weak and which handles a video/audio information file (hereinafter, simply abbreviated as a video file) preserved or stored in a recording medium of a large capacity. According to the former system, since the coding on the transmission side of the video and audio signals and the decoding on the reception side must be performed in a real-time manner, an exclusive-use hardware is needed. The latter system relates to, for example, the case of constructing a video data base comprising a number of video files. More importance is paid to picture quality, a transmitting time (or transmission costs), or the like than is paid to the real-time performance of the video image.
There are the following limitations in a video/audio transmitting system which is constructed by using a general work station. That is, in general, a frame memory of the display system is constructed by eight bits and uses 256 colors among 6.40 million colors. The color to be displayed can be selected by a lookup table method. On the other hand, in the case where a black and white image and a color image mixedly exist and are displayed by a color monitor of eight bits, the black and white image can be expressed by 128 gradations (gray scale) and a color image can be expressed by 64 colors by allocating two bits to each of the colors of R, G, and B by a dither processing.
In communicating and accumulating systems, both of the audio data and the video data are compressed and coded, thereby reducing a data amount. As a compressing method, there are three types of methods: an interframe coding (INTER) method; an intraframe coding (INTRA) method; and a method using both of the INTER method and the INTRA method. According to the compressing method using both of the interframe coding method and the intraframe coding method, generally, in order to prevent the propagation of a transmission error and to make a special reproduction easy, the intraframe coded frames are arranged adaptively or uniformly at proper intervals. As an individual coding method, a discrete cosine transformation (DCT), a differential pulse code modulation (DPCM), or the like is used.
Although it is sufficient to use the interframe coding many times in order to raise a compression ratio, there is a drawback such that the data of the previous frame is necessary for decoding.
In a moving image, it is necessary to synchronize the reproduction of the video image and the reproduction of the audio signal. However, in the case where a processing ability of the hardware or a processing ability in reproducing processing is low, it is necessary to skip proper frames. In the conventional apparatus, even when performing such a frame skip, the interframe coded frame is once decoded and the system is set into a state in which the system is prepared for the decoding of the next frame and, after that, the frame skip is executed, so that it takes a long time for decoding. Consequently, the number of frames which must be skipped increases and a reproduction image becomes unnatural.
In the above-mentioned image communication system, however, when a CPU executes processing to reproduce and display a moving image,
(1) in the case where another process is being performed by a multitask function, or PA1 (2) in the case where the ability of the CPU is inferior to the performance that is expected on the transmission side, or the like, there is a drawback such that it is impossible to synchronously reproduce all of the frames while all of the frames are displayed.
That is, many general computers have a multi task function such that a plurality of processes can be simultaneously executed by the general computer. Since computer resources such as memory capacity which can be used in the processing to reproduce a moving image, processing ability of the CPU, and the like change momentarily, according to a conventional algorithm for the synchronous reproduction using the processing by the exclusive-use hardware as a prerequisite, there are problems such that a proper synchronization cannot be performed and the synchronous reproduction can be made impossible.
On the other hand, there is a TV telephone in an image communicating system having a low bit rate. In the TV telephone system, the transmission and reception of data in a real-time manner are fundamental and a hardware such as exclusive-use terminal, exclusive-use expansion board, and the like are necessary. On the other hand, a system in which a video file is exchanged on a computer such as a general work station or the like is also known. According to such a method using the video file, in association with the improvement of a processing speed of the CPU, the processings such as communication, play display, or the like of an image can be processed by software. However, there is also a problem such that synchronous reproduction is also made impossible in a manner similar to the foregoing case.